Silicones are synthetic polymers based on chains or networks of alternating silicon and oxygen atoms. Also called polymerized siloxanes or polysiloxanes, silicones have the general chemical formula [R2SiO]n, where R is an organic group such as methyl, ethyl, or phenyl. Chemically, these materials have an inorganic silicon-oxygen backbone ( . . . —Si—O—Si—O—Si—O— . . . ) with organic side groups attached to the silicon atoms. Silicones are generally known for their uses as electrical insulators, waterproofing agents, rubbers and resins.
Foams may have either an open-celled or closed-celled microstructure. Open-cell foams have voids that generally intersect one another, forming paths that percolate through the material. These foams tend to be soft and compressible compared to closed cell foams. Open-cell foams are also generally lower density than closed cell foams and are somewhat less expensive, since less material is needed per unit volume. Closed cell foams, on the other hand, use isolated voids that do not communicate with each other. As such, these foams tend to be stronger and resist compression. Moreover, gases and fluids cannot penetrate through closed cell foams, making closed cell foams useful in certain insulation applications where absorption of water would not be desirable.
Various methods can be used to prepare foamed polymers. Some involve use of a physical or chemical blowing agent. Physical blowing agents are generally volatile liquids that can be used to create voids in a matrix, thereby producing a cellular (or foamed) material. Common physical blowing agents include chlorofluorocarbons, hydrochlorofluorocarbons, hydrocarbons and liquid carbon dioxide. Chemical blowing agents expand the foam using one or more chemical reactions that produce a gas. An exemplary chemical blowing agent is powdered titanium hydride, which can be used to make metallic foams by decomposing into titanium and hydrogen gas at elevated temperatures. Other foaming methods have also been reduced to practice, including mechanical frothing, extraction of soluble fillers, and use of hollow sphere and/or porous particles.
Silicone materials in general have low thermal conductivity, low chemical reactivity, low toxicity, good oxidation stability and good thermal stability (i.e. their properties do not significantly change over a wide temperature range). These properties make them particularly useful in thermo-acoustic, electrical insulation, and sealing applications that demand high temperature resistance, low smoke generation, and/or high biocompatibility. These include, for example, medical applications and transportation applications in aircraft, ships, and railway systems. Silicone foams also offer potential cost savings based on their reduced material cost when compared with solid silicone elastomers.